1. Field of the Invention
The present invention relates to a power supply device which can detect an open load through electromagnetic induction.
2. Description of the Related Art
In general, since LCD products such as LCD TVs or LCD monitors cannot emit light by themselves, a backlight device for providing light to an LCD panel is used therein. Such a backlight device provides light by using a plurality of discharge lamps. The resistance value of the respective lamps when the backlight device is initially driven is different from that of the lamps when the backlight device is normally driven. Therefore, when the lamps are driven in parallel, a current balance should be maintained using a current balancing transformer.
Further, when the lamps adopted in the backlight device are driven in parallel, the brightness of the lamps should be constantly maintained and adjusted. For this, a power supply unit of the backlight device is provided with a feedback circuit and a protection circuit. The feedback circuit receives the currents of the lamps to constantly maintain the lamp currents, and the protection circuit protects the lamps and the power supply unit when an excessive voltage is applied to the lamps.
Hereinafter, a conventional power supply device will be described with reference to FIG. 1.
FIG. 1 is a circuit diagram of a conventional power supply device.
As shown in FIG. 1, the conventional power supply device includes a power supply unit 110, a current balancing unit 120, a detection unit 130, and a control unit 140 and drives a plurality of loads L1 to L6 with constant brightness.
The power supply unit 110 is connected to the plurality of loads L1 to L6, the current balancing unit 120, and the control unit 140 and is controlled by the control unit 140 so as to output a driving voltage Vin for driving the plurality of loads L1 to L6.
The current balancing unit 120 is composed of a plurality of transformers T1 to T6 having primary and secondary sides. The current balancing unit 120 receives the driving voltage Vin output from the power supply unit 110 so as to balance currents of the driving voltage Vin. Then, the current balancing unit 120 supplies the driving voltage with a constant magnitude to the respective loads L1 to L6.
The number of the transformers T1 to T6 is equal to the number of the loads L1 to L6 such that the transformers T1 to T6 are connected to the respective loads L1 to L6 one by one. The primary sides of the transformers T1 to T6 receive the driving voltage Vin to supply to the respective loads L1 to L6. Further, the secondary sides thereof are connected in series to each other so as to maintain a current balance of the driving voltage Vin applied from the primary sides.
The detection unit 130 is connected to the current balancing unit 120 and the control unit 140 and includes a plurality of voltage dividing sections 131 to 136, first to sixth diodes D1 to D6, and a capacitor C0. The detection unit 130 detects a current flowing in the current balancing unit 120 so as to output a detection signal P corresponding to the detected current.
At this time, the plurality of voltage dividing sections 131 to 136 are connected to the secondary sides of the respective transformers T1 to T6 and respectively have two resistors. The voltage dividing sections 131 to 136 receive and divide a voltage corresponding to the current flowing in the connected transformers.
The first to sixth diodes D1 to D6 receive and output the voltages divided by the plurality of voltage dividing sections 131 and 136, and the capacitor C0 smoothes the divided voltages applied through the first to sixth diodes D1 to D6 to output as a detection signal P.
The control unit 140 is connected to the detection unit 130 and the power supply unit 110. When the detection signal P delivered through the detection unit 130 is larger or smaller than a preset reference voltage, the control unit 140 judges that one or more of the loads L1 to L6 are opened or short-circuited. Then, the control unit 140 outputs a control signal S for controlling the power supply unit 110.
Accordingly, when the loads are opened or short-circuited, the control unit 140 controls the power supply unit 110 so as to control the output of the driving voltages Vin is controlled. Then, it is possible to protect the plurality of loads L1 to L6 and the power supply device.
However, the conventional power supply device has the following problems.
In the conventional power supply device, one transformer should be provided to drive one load. In FIG. 1, six of the transformers T1 to T6 should be provided in the current balancing unit 120 to drive six of the loads L1 to L6. Therefore, as the number of loads increases, the volume of the power supply device increases, and the circuit becomes complex.
Further, the detection unit 130 of the conventional power supply device is directly connected to the plurality of transformers T1 to T6 required for insulation design. Therefore, there are difficulties in applying the detection unit 130 to a load requiring a high voltage.